Recombinant MafA protein containing its own protein transduction domain stimulates insulin gene expression in IEC-6 cells

Aims MafA, a basic leucine zipper (bZIP) transcription factor, functions as a potent activator of insulin gene transcription in β-cell. In this paper, we aimed to investigate whether the entire MafA protein has the self-delivery activity, and that the arginine- and lysine-rich sequence in MafA bZIP domain is an efficient protein transduction domain (PTD). Main methods Entire MafA protein internalization was evaluated with Western blot and immunofluorescence. The distribution of the PTD-EGFP (enhanced green fluorescence protein) was examined by fluorescent microscope observation. Luciferase reporter assay was used to investigate the effect of the transduced MafA protein on insulin 2 promoter activity. Additionally, we conducted RT-PCR to detect the expression of insulin mRNA in MafA treated IEC-6 cells. Key findings The arginine- and lysine-rich peptide of MafA serves as a novel PTD. PTD-EGFP can permeate into various cell types including Min6 (a β-cell line), and transduce in a dose- and time-dependent manner. The cellular uptake of MafA PTD can be completely blocked by heparin, whereas cytochalasin D and amiloride were partially effective in blocking the PTD-EGFP protein entry. Transduced intact MafA protein behaves in the same way as the endogenous MafA, stimulating the transcription of insulin promoter and further inducing insulin expression in treated non-β-cell line (IEC-6). Significance These results indicate that the MafA PTD could serve as a therapeutic delivery vehicle, and further suggest that MafA protein transduction could be a valuable strategy for enhancing insulin gene transcription without requiring gene transfer technology.